1. Field of the Invention
The present invention relates to a nested modulator where sub-Mach-Zehnder waveguides are incorporated into the two branching waveguides of the main Mach-Zehnder waveguide.
2. Description of the Related Art
In conventional optical communication technology and optical measurement technology, SSB (single side band) modulators, which are a type of nested modulators where sub-Mach-Zehnder waveguides (hereinafter, referred to as “sub-MZ waveguides”) are incorporated into the two branching waveguides of the main Mach-Zehnder waveguide (hereinafter, referred to as “main MZ waveguide”), are used. As disclosed in the following Patent Document 1, these make it possible to suppress a specific spectrum of the modulated light (discrete spectrum of frequency ω+nΩ, where n is an integer) emitted from the main MZ waveguide by introducing, for example, light waves having frequency ω into the main MZ waveguide 1, and at the same time, applying an RF signal (frequency Ω) RFA having a single frequency and a signal RFB which is gained by carrying out Hilbert conversion on this RF signal to the two sub-MZ optical waveguides 2 and 3, and thus, adjusting the DC biases DCA, DCB and DCC, which are applied to the main MZ waveguide and the respective sub-MZ waveguides (see FIG. 1). In addition, the nest type modulators include an FSK modulator and a DQPSK modulator where an RF signal RFC is applied instead of the DC bias DCC of FIG. 1.
Patent Document 1: Japanese Unexamined Patent Publication 2004-245750
In the case where an X cut plate is used as the substrate of a nested modulator as shown in FIG. 2, however, signal electrodes, which are modulating electrodes, are formed in locations 15 and 16 on the sub-branching waveguides 11 and 12 as well as 13 and 14 of the respective sub-MZ waveguides 2 and 3 on the substrate 10, and furthermore, ground electrodes are formed in locations 17, 18 and 19. Here, FIG. 2 is a cross sectional diagram showing the nested modulator of FIG. 1 along one dot chain line A.
In the case of FIG. 2, only a single signal electrode is formed for each sub-MZ waveguide, and therefore, the waveguides and the signal electrodes are at a distance from each other, and thus, there is a disadvantage in that the drive voltage becomes high though there is an advantage in that the structure becomes relatively simple.
In contrast, in the case where a Z cut plate is used as the substrate of a nested modulator as shown in FIG. 3, it becomes possible to locate signal electrodes 26, 27, 28 and 29 in close proximity to each of the sub-branching waveguides 21 and 22 as well as 23 and 24 of the respective sub-MZ waveguides 2 and 3 on the substrate 20, and therefore, it becomes possible to lower the drive voltage. Here, 25 indicates a buffer layer.
However, it is necessary to always secure two signal electrodes for each of the sub-MZ waveguides 2 and 3, making complicated the wiring of the signal electrodes, and in addition, it is necessary to apply a modulating signal in the opposite phase to each of the sub-branching waveguides, and therefore, it is very difficult to adjust the length of the signal electrode between the signal entrance and the portion in which the electrical field created by the electrode works on the optical waveguide.
An object of the present invention is to solve the above described problems and provide a nested modulator where the circuit arrangement of the modulating electrodes including signal electrodes is simplified, and it is made possible to lower the drive voltage.